Chaos reigned at the grand opening of Six Flags Over Georgia … with mobs of unruly teenagers fighting inside the theme park, escalating to a massive police response and an officer-involved shooting.

Cops in Georgia say there were 500 to 600 youth running amok in the park Saturday night, getting out of control and leading to a huge brawl and panic among park guests.

The huge crowd of fighting teens overwhelmed theme park security, who called police.

When cops got there, officers say they followed the group out of Six Flags onto a nearby service road … where gunshots started ringing out.

Police say several suspects fired at officers, with cops returning fire. One person was shot, reported to be a 15-year-old … who’s said to be in critical condition.

Six Flags Over Georgia confirmed a shooting outside the grounds and told FOX 5 Atlanta … “It is incredibly disappointing that our community is disrupted at public events throughout the region by groups of underaged youth.”

The park adds … “We won’t put up with that type of activity here.”

Meanwhile, the Georgia Bureau of Investigation is investigating the officer-involved shooting.